Cubic boron nitride (hereinafter, also referred to as “cBN”) has hardness second to diamond and is also excellent in thermal stability and chemical stability. Further, cubic boron nitride is more stable with respect to an iron-based material than diamond, so that a cBN sintered material has been used as a working tool for the iron-based material.
However, the cBN sintered material includes about 10 to 40 volume % of a binder, and this binder causes reduction of the strength, heat resistance, and thermal diffusion property of the sintered material. Therefore, particularly when cutting an iron-based material at a high speed, thermal load becomes large and the cutting edge is likely to be chipped and cracked, resulting in a short life of the tool.
As a method of solving this problem, there is a method of producing a cBN sintered material using a catalyst without using a binder. In this method, reaction sintering is performed using hexagonal boron nitride (hBN) as a raw material and using magnesium boron nitride (Mg3BN3) or the like as a catalyst. The cBN sintered material obtained by this method does not include a binder, so that cBN grains are bonded to each other strongly and thermal conductivity becomes high. Therefore, the cBN sintered material is used for a heat sink material, a TAB (Tape Automated Bonding) bonding tool, or the like. However, because a small amount of the catalyst remains in the sintered material, a fine crack is likely to be caused under application of heat due to a difference in thermal expansion between the catalyst and the cBN and the cBN sintered material is therefore not suitable for a cutting tool. Moreover, because the grain size is large, specifically, about 10 μm, the thermal conductivity is high but the strength is weak and the cBN sintered material is therefore incapable of applications for cutting involving a large load or the like.
On the other hand, a cBN sintered material can be also obtained by directly converting normal pressure type BN (boron nitride) such as hBN into cBN without using a catalyst under ultra-high pressure and high temperature and by sintering it at the same time (direct conversion sintering method). For example, each of Japanese Patent Laying-Open No. 47-034099 (Patent Document 1) and Japanese Patent Laying-Open No. 03-159964 (Patent Document 2) describes a method of converting hBN into cBN under ultra-high pressure and high temperature so as to obtain a cBN sintered material. Moreover, there is a method of obtaining a cBN sintered material using pyrolytic boron nitride (pBN) as a raw material. This type of method is illustrated in, for example, in Japanese Patent Laying-Open No. 54-033510 (Patent Document 3) or Japanese Patent Laying-Open No. 08-047801 (Patent Document 4). In this method, conditions such as 7 GPa and not less than 2100° C. are required.
Each of Japanese Examined Patent Publication No. 49-027518 (Patent Document 5) and Japanese Patent Laying-Open No. 11-246271 (Patent Document 6) describes a method of obtaining a cBN sintered material under conditions less strict than the above-described conditions.